Window boxes add charm and quaintness to buildings, softening the harsh lines of the structure while bringing color and foliage. Since window boxes are containers of soil that are exposed to fast-drying, wind-exposed and extremes in hot and cold temperatures, year-round plants must be adaptable to these growing conditions.
Evergreens, those that are with needles or broad leaves, make for good year-round components in a window box, provided they are well-adapted for the growing conditions. In USDA hardiness zones 1-5 there are few evergreen plants besides creeping junipers (Juniperus spp.) that will tolerate both the bitter winter cold and the heat of summer. In warmer regions, zones 6-9, junipers, jasmines (Trachelospermum spp.) and ivies (Hedera spp.) can be used, as well as wintercreeper (Euonymus japonicus), as the mild winters will not dry out foliage readily. In subtropical areas, zones 9 to 12, even more plants can be used year round, as the lack of a killing frosts allows many plants to remain presentable in all seasons.
Since window boxes are generally long and narrow in shape, there is limited soil for plants to grow into and sustain themselves. The shallow soil drains quickly after watering and with potential exposure to reflected light off of the building facade, the soil may become dry within hours. Drought-tolerant plants are a good choice for containers, as they will not falter as quickly as many annual flowers that appreciate a cool, moist soil at all times.
Some examples of drought tolerant plants include junipers, hens-and-chicks (Sempervivum spp.), stonecrops (Sedum spp.) and periwinkle (Catharanthus roseus). In shady boxes, barrenwort (Epimedium spp.) and bromeliads and crown-of-thorns (Euphorbia spp.) are good examples; the latter two only in frost-free areas.
Also take into consideration that winds are heightened around window boxes, and act to dehydrate plants more quickly. Protected windows, or those on shady sides of buildings have a diminished exposure to the drying effects of wind, although still occur. In winter the cooler temperatures coupled with wind can negatively effect plants, regardless of region.
With an exposed location, the soil of the window box will fluctuate more frequently than the soil in the ground. Thus, on frosty nights, there is no radiant heat from the ground keeping the soil warmer. Although heat from the building can keep the window box slightly warmer, the box will more readily cool down to temperatures that equal air temperatures. Plants that are tolerant of cold and survive cold winters are advantageous, as they will have to endure colder winters than those nearby that are growing in soil.
In like manner, the soil in window boxes will heat up more quickly than that of the ground, adding stress to plants, especially if moisture is lacking. The reflected sunlight or radiant heat eminating from the sun-warmed building facade will keep the window box soil warmer, including into the evening and night hours.
Use of Seasonal Plants
With variations in temperatures and wind across the year, gardeners always rely on seasonal annual and perennial plants to bring color and texture to the window box composition. Even the window box with a hardy, year round plant in it benefits visually with a fresh, seasonal addition of color and vigor.
Seasonal plants are used based on their tolerance for growing conditions in the window box. For examples, pansies (Viola spp.) are best used in cool weather, and often grace containers from fall to spring. Conversely, heat and drought tolerant plants are used in summer, such as lantana, petunia, or starcluster (Pentas spp.). Once the seasonal plants' performances wane as weather conditions change, they are removed and replaced.